CSS parsing is 1-2% of WebKit CPU usage on average pages, more on sites with large stylesheets.
We currently reparse all stylesheets from source text when they are encountered again. In many
browsing scenarios we can eliminate lot of this by caching the parsed stylesheets. For example
it is very common for subpages of a site to share the stylesheets.

This patch enables memory caching for stylesheet loaded using the <link> element. Only stylesheets
that have no import rules are cacheable for now.

Cached stylesheets are copied on restore so there is no sharing (and no memory wins) yet.
In the future we will also be able to share the actual data structures between pages for
significant memory savings.

After browsing around for a while <5% of the memory cache data was in parsed stylesheets so this
does not bloat the cache significantly.